The palm sweat meter (prove that holding hands just isn’t that bad, the scientific way). Should be doable using the humidity sensor in the new Galaxy S4.

A game that simulates real physics. It will be dull but someone should make it just so it’s been done.

Someone should properly figure out all the mDNIe fields so Samsung’s oversaturated OLED travesty can be somewhat improved. I did figure out most of them on the Galaxy S2 but lacked a colorimeter at the time so I couldn’t write a simple display calibrator. It is possible to mostly fix color temperature and gamma (same curve shared for all RGB). 10K+ color temperature makes me blue.

An IR diod/front camera, so the useless Samsung face stay and rotate features can work when it’s not perfect lighting conditions. Like in bed.

Or to take the previous one even further. IR camera+laser projected IR pattern. Then we could have kinect style 3D scanning in our phone. Don’t think I’m crazy, if you can stick a projector into a phone this can be done too.